Aircraft-based Enhanced Vision Systems (EVS) may experience difficulty detecting approach lighting systems (ALS), airport markings, and other important indicators of an airport environment. For example, EVS systems rely on the detection of radiation in spectral ranges outside the visible band. Conventional approaches to ALS incorporate incandescent lighting which emits significant portions of energy in the short-wave infrared (SWIR) band. EVS detection in the SWIR band is advantageous due to the relative advantage of atmospheric penetration of SWIR radiation (relative to both atmospheric penetration of visible-band radiation and background illumination due to solar radiation). Both visible-band and background atmospheric radiation compete with desirable ALS emissions for detection by onboard cameras or image sensors.
The background illumination competing with desirable signals for detection can be especially problematic in daylight conditions. For example, bright sunlight may illuminate a water-laden atmosphere (e.g., fog) resulting in a bright “airtight” background that easily saturates many modern image sensors, complicating or preventing the detection of less intense point sources (e.g., desirable ALS emissions) within this background illumination. Furthermore, modern ALS and other airfield lighting systems may incorporate light emitting diode (LED) based illumination in order to save energy and maintenance costs compared to conventional incandescent lighting systems. However, LED-based illumination systems often do not emit significant amounts of radiation outside the visible band, rendering ALS detection in alternative and advantageous spectral bands near impossible in daylight or “airtight” conditions.
A potential solution involves overwhelming background illumination by increasing the brightness of visible-band ALS emissions far beyond regulatory requirements (generally 5,000-30,000 cd) in order to drown out competing background illumination. However, the energy required for this approach may render it cost-prohibitive and result in brightness inappropriate for pilots.